780 

14 CFR Ch. I (1–1–19 Edition) 

Pt. 36, App. A 

A36.3.4.1 In the absence of wind and for si-

nusoidal sounds at grazing incidence, the in-
sertion loss caused by the windscreen of a 
stated type installed around the microphone 
must not exceed 

±

1.5 dB at nominal one-third 

octave midband frequencies from 50 Hz to 10 
kHz inclusive. 

A36.3.5 

Microphone system. 

A36.3.5.1 The microphone system must 

meet the specifications in sections A36.3.5.2 
to A36.3.5.4. Various microphone systems 
may be approved by the FAA on the basis of 
demonstrated equivalent overall 
electroacoustical performance. Where two or 
more microphone systems of the same type 
are used, demonstration that at least one 
system conforms to the specifications in full 
is sufficient to demonstrate conformance. 

N

OTE

: An applicant must still calibrate 

and check each system as required in section 
A36.3.9. 

A36.3.5.2 The microphone must be mounted 

with the sensing element 4 ft (1.2 m) above 
the local ground surface and must be ori-
ented for grazing incidence, 

i.e.

, with the 

sensing element substantially in the plane 
defined by the predicted reference flight path 
of the aircraft and the measuring station. 

The microphone mounting arrangement 
must minimize the interference of the sup-
ports with the sound to be measured. Figure 
A36–1 illustrates sound incidence angles on a 
microphone. 

A36.3.5.3 The free-field sensitivity level of 

the microphone and preamplifier in the ref-
erence direction, at frequencies over at least 
the range of one-third-octave nominal 
midband frequencies from 50 Hz to 5 kHz in-
clusive, must be within 

±

1.0 dB of that at the 

calibration check frequency, and within 

±

2.0 

dB for nominal midband frequencies of 6.3 
kHz, 8 kHz and 10 kHz. 

A36.3.5.4 For sinusoidal sound waves at 

each one-third octave nominal midband fre-
quency over the range from 50 Hz to 10 kHz 
inclusive, the free-field sensitivity levels of 
the microphone system at sound incidence 
angles of 30

°

, 60

°

, 90

°

, 120

° 

and 150

°

, must not 

differ from the free-field sensitivity level at 
a sound incidence angle of 0

° 

(‘‘normal inci-

dence’’) by more than the values shown in 
Table A36–1. The free-field sensitivity level 
differences at sound incidence angles be-
tween any two adjacent sound incidence an-
gles in Table A36–1 must not exceed the tol-
erance limit for the greater angle. 

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